Such a speed control device can be utilized, for example, for an additional heating apparatus of a combustion engine of a vehicle, with the combustion engine driving a blower for supplying the amount of air necessary for the fuel-air mixture. For obtaining an optimum combustion of the heater fuel, the ratio X between air and fuel of the fuel mixture should be kept constant. This means that a constant fuel amount per unit of time and a constant amount of air per unit of time have to be supplied for producing the fuel-air mixture. In order to keep constant the supplied amount of air per unit of time, the combustion engine or blower motor must be kept at a constant speed. However, this presupposes a constant battery voltage of the vehicle battery powering the additional heating apparatus of the vehicle.
In practical applications, the battery voltage of a vehicle may show relatively large fluctuations. In case of a vehicle battery having a nominal voltage of 24 V, as used for example for lorries/strucks or busses, a fluctuation width or range of the battery voltage between 18 V and 32 V is assumed. This results in a correspondingly high fluctuation of the speed of the blower motor and in a corresponding fluctuation of the amount of air supplied per unit of time for preparing the fuel-air mixture.
For obtaining a constant blower speed despite these high fluctuations in battery voltage, it is common practice to keep the speed of the blower motor at a constant set value by means of a speed regulating circuit. Such a regulating circuit necessitates a sensor disposed on the electric motor or on the blower impeller driven by the latter and serving for measuring the actual speed value. To this end, optical sensors or magnetic sensors are usually employed. Both sensor types increase the manufacturing costs and entail problems. Optical sensors are subject to the problem of soiling. Magnetic sensors are subject to the problem that they must be disposed as close as possible to a magnetic generator.